The background of the invention is discussed briefly in the following.
Video Interactive Gateway (VIG) makes it possible for interworking between low bit-rate multimedia terminals (H.324) located in circuit switched telephony network and terminals in IP based multimedia systems (H.323).
For a better understanding of the prior art relation to the present invention reference will be made to the accompanying figures in which:
FIG. 1 illustrates an Schemantical Diagram of H.324-H.323 interworking according to prior art.
FIG. 2 illustrates Uni-directional Logical Channel signalling procedures according to prior art.
FIG. 3 illustrates Bi-directional Logical Channel signalling procedures according to prior art.
FIG. 4 illustrates Interworking between Bi-directional Logical Channel signalling procedures and Uni-directional Logical Channel signalling procedures according to prior art.
FIG. 1 illustrates an Schemantical Diagram of H.324-H.323 interworking according to prior art. The circuit switched network may use the 64 kbit/s unrestricted digital bearer for the multimedia connection.
Using H.223 as multiplexing protocol different multimedia components, audio, video and data are multiplexed within the bearer. These channels are de-multiplexed by VIG to separate RTP and TCP channels in the IP network, and vice versa. VIG may perform transcoding for different multimedia components if needed to make the communication between end-terminals possible.
ISUP may be used in the circuit switched network as a call control protocol, which is mapped to H.225 protocol in the IP network by VIG.
H.245 is used as a control protocol both in the circuit switched network and in the IP network, providing end to end capability exchange, signalling of command and indications, and messages to open and describe the content of logical channels for different multimedia components.
VIG performs mapping of H.245 messages between circuit switched network and IP network, in order to adapt the different transport protocols and to enable transcoding of media channels.
Further, the interworking capabilities can be used for enabling IP trunking, in case of H.324 to H.324 call. This means that the call is routed to IP-network as close to the terminals as possible. This makes it possible to utilize cheaper transmission costs in the IP network compared to the transmission costs in the ordinary circuit switched telephony network.
As specified in H.245 Open Logical Channel signalling procedures are used for establishing a media channel between terminals. These procedures actually consist of two different procedures, i.e. Uni-directional Logical Channel signalling procedures and Bi-directional Logical Channel signalling procedures.
Uni-directional Logical Channel signalling procedures are used for opening a one-way media channel between the terminals. The originating terminal, which is going to transmit media through the channel, initiates the procedure by issuing Open Logical Channel (OLC) message to remote terminal. The OLC contain description of the media type, the codec to be used, codec options to be used and description of the multiplex layer options to be used, for the channel being opened.
The remote terminal checks if it is able to comply the description of the channel as a receiver. If that is the case it responds with the Open Logical Channel Acknowledge message. When the originating terminal receives the message it can start to transmit the media according to the protocol negotiated in OLC signalling procedure.
FIG. 2 illustrates Uni-directional Logical Channel signalling procedures according to prior art. Bi-directional Logical Channel signalling procedures are used for opening a two-way media channel between the terminals. The channel consists of two uni-directional channels, i.e. forward channel and reverse channel.
The originating terminal, which is going to transmit and receive media through the channels, initiates the procedure by issuing Open Logical Channel (OLC) message to remote terminal. The OLC contain description of the media type, the codec to be used, codec options to be used and description of the multiplex layer options to be used, for the channels being opened both for forward channel i.e. the media which the originating terminal is going to transmit, and for the reverse channel, i.e. for the media which the originating terminal is going to receive.
The remote terminal checks if it is able to comply the description of the channels as a receiver for the forward channel and as a sender for the reverse channel. If that is the case it responds with the Open Logical Channel Acknowledge message.
When the originating terminal receives the message it issues Open Logical Channel Confirm and can start to transmit the media on the forward channel according to the protocol negotiated in OLC signalling procedure. When the terminating terminal receives the message it can start to transmit the media on the reverse channel according to the protocol negotiated in OLC signalling procedures.
Further, H.245 specifies the master slave determination procedure in order to find, which terminal will act as a master and which terminal will act as a slave during the call. This procedure happens in the beginning of the call in order to be able to solve conflicts at logical channel signalling procedure, e.g. both terminals start Bi-directional Logical Channel signalling procedures simultaneously.
FIG. 3 illustrates Bi-directional Logical Channel signalling procedures according to prior art. H.323 specifies that Uni-directional channels are used for audio and video channels. This means that the both terminals are supposed to initiate a uni-directional logical channel signalling procedure to open a channel for the media, which they intend to transmit. H.324 specifies the same for the audio channels. However, it is required that it is mandatory for H.324 terminal to support H.223 adaptation layer 3 for video channel. Since H.223 adaptation layer 3 has bi-directional behaviour, H.324 requires that bi-directional logical channel signalling procedure is used for opening the video channels. In addition to AL3, H.324 terminal may support video channels by using H.223 AL2, which enables to use uni-directional channels.
H.246 specifies the interworking between H.323 and H.324. For Logical Channel Signalling it specifies: “When OpenLogicalChannel (OLC) messages (either unidirectional or bi-directional) are received from the remote endpoint (H.323 or H.324), the gateway should initiate the OLC procedure at the opposite endpoint of the gateway. The Gateway may initiate the OLC procedure before receiving OLC messages.”
The problems arising from the background solutions are discussed briefly in the following.
In case call is made between H.324M terminal, which does not support H.223 AL2 for video channels, to H.323 terminal, VIG needs to map bi-directional logical channel signalling procedures used on H.324 side to the uni-directional logical channel signalling procedures used on the H.323. However, this causes potential problems for VIG as seen in FIG. 4.
FIG. 4 illustrates Interworking between Bi-directional Logical Channel signalling procedures and Uni-directional Logical Channel signalling procedures according to prior art. Mapping of the procedures creates two fundamental problems.
VIG receives two descriptions for the channel in direction from H.323 terminal to H.324, one from H.324 terminal as reverse channel description, and another from H.323 terminal as forward channel description. Since the fundamental goal is that VIG do not make any transcoding of adaptation on media when ever possible, this situation creates a problem, when the descriptions received from the different terminals do not match with each other. Further, since different procedures are used for opening of the logical channels, there is no straightforward way to relay terminals to negotiate the used description.
OLC ack and OLC conf messages trigger terminals to start transmission of media. It must be guaranteed that the procedures on the both side of VIG are synchronized so that when VIG receives media from a terminal it is able to transfer media forward to another terminal, in order not to loss the media, since this would be hard to recover by terminals for video channels. It has been noticed that some terminals will crash.
There is a clear need for a solution for interworking between a number of video communication terminals that would make it possible to interwork between different terminals e.g. between a low bit-rate multimedia terminal and a packet based multimedia terminal.